#define _CRT_SECURE_NO_WARNINGS
#pragma warning(disable : 4996)
#pragma warning(disable : 6031)

#include <stdio.h>

int main()
{
    int a = 0;
    int b = 0;
    int c = 0;
    int d = 0;
    scanf("%d %d %d %d", &a, &b, &c, &d);
    printf("%d", (a + b - c) * d);
    return 0;
}